The permanent magnet synchronous drive system, as a new type of drive system, is used to control a rotation speed and a rotation torque of a permanent magnet synchronous motor in a frequency modulation and voltage modulation way, and has become an important integral component of an alternating-current drive system due to its advantages such as a simple structure, a great power density, great output torque at a low speed, a high efficiency and convenient maintenance. In an application of the permanent magnet synchronous drive system, an operating technology at a speed refers to a process in which the permanent magnet synchronous motor performs a powered operation again while the motor still has a certain initial speed after being powered off. For example, in the field of rail transit, the motor needs to perform traction again at a speed in a situation such as passing neutral section and short-time overcurrent and overvoltage. Further, for example, in the field of wind power generation, a motor having a certain initial speed needs to operate again before a converter starts to operate. In each of these operating conditions, the permanent magnet synchronous drive system needs to have the ability to operate at a speed.
The permanent magnet synchronous motor has a certain initial speed, which means that an induced potential may be induced on a stator. In a case that the permanent magnet synchronous motor is initiated directly without knowing a rotor position, a strong current and mechanical shock is caused unavoidably, and an overcurrent fault is caused in the converter, resulting in the converter becoming un-operable. It is of great significance to develop a technology for observing an initial position of the permanent magnet synchronous motor having the ability to operate at a speed.
To solve the problem described above, a way is to detect information on a rotor position by a position sensor installed at a bearing of the rotor of the motor. However, the position sensor will not only increase the volume and the cost of the motor, but also has a strict demand for usage environment, since external environmental factors such as vibration or humidity have a great effect on the position sensor. Another way is to install a voltage sensor at the stator for sampling the induced potential of the permanent magnet synchronous motor in a real-time manner before the converter starts to operate, so as to obtain the information on the rotor position. In this case, although the position sensor at the rotor is not required, the voltage sensor is required, which also increases the cost. In order to overcome the drawbacks caused by these sensors, another way to determine the rotor position is to use a control algorithm without a sensor as follows.
A voltage equation of the permanent magnet synchronous motor in a two-phase d-q rotating coordinate system is:
                              [                                                                      v                  d                                                                                                      v                  q                                                              ]                =                                            [                                                                                          R                      +                                              pL                        d                                                                                                                                                                          -                          ω                                                ⁢                                                                                                  ⁢                                                  L                          q                                                                    ⁢                                                                                                                                                                                                                                                                          ⁢                                              ω                        ⁢                                                                                                  ⁢                                                  L                          d                                                                                                                                                R                      +                                              pL                        q                                                                                                        ]                        ⁡                          [                                                                                          i                      d                                                                                                                                  i                      q                                                                                  ]                                +                                    ωφ              f                        ⁡                          [                                                                    0                                                                                        1                                                              ]                                                          (        1        )                            where ω is an angular velocity of the rotor, R is a resistance of the stator, φf is a permanent magnet flux linkage, id and iq are a d-axis current and a q-axis current respectively, vd and vq are a d-axis voltage and a q-axis voltage respectively, Ld and Lq are a d-axis inductance and a q-axis inductance respectively, and p is a differential operator.        
All upper bridge arms (or lower bridge arms) of the inverter are switched on, so that the three-phase winding of the permanent magnet synchronous motor is short-circuited for a short time period. In this case, a voltage on the stator is equal to 0, that is, vd=0 and vq=0, thus the voltage equation is:
                              [                                                    0                                                                    0                                              ]                =                                            [                                                                                          pL                      d                                                                                                                          -                        ω                                            ⁢                                                                                          ⁢                                              L                        q                                                                                                                                                        ω                      ⁢                                                                                          ⁢                                              L                        d                                                                                                                        pL                      q                                                                                  ]                        ⁡                          [                                                                                          i                      d                                                                                                                                  i                      q                                                                                  ]                                +                                    ωψ              f                        ⁡                          [                                                                    0                                                                                        1                                                              ]                                                          (        2        )            
Assumed that Tsh is a short-circuit time period, ω is a constant, the resistance of the stator is ignored, an initial current is 0, a short-circuit current vector on the stator is obtained by transforming the equation (1):
                              i                      (                          T              sh                        )                          =                              [                                                                                i                                          d                      ⁡                                              (                                                  T                          sh                                                )                                                                                                                                                              i                                          q                      ⁡                                              (                                                  T                          sh                                                )                                                                                                                  ]                    =                      [                                                                                                      -                                                                        ψ                          f                                                                          L                          d                                                                                      ⁢                                          (                                              1                        -                                                  cos                          ⁡                                                      (                                                          ω                              ⁢                                                                                                                          ⁢                                                              T                                sh                                                                                      )                                                                                              )                                                                                                                                                                                      ψ                        f                                                                    L                        q                                                              ⁢                                          sin                      ⁡                                              (                                                  ω                          ⁢                                                                                                          ⁢                                                      T                            sh                                                                          )                                                                                                                  ]                                              (        3        )            
The short-circuit current is transformed into a α-β static coordinate system, and a phase angle of the short-circuit current in the α-β static coordinate system is obtained:
                              θ                      I            ⁢                                                  ⁢            αβ                          =                  arctan          ⁡                      (                                          i                α                                            i                β                                      )                                              (        4        )            
An initial angular frequency ωe of the stator is calculated according to a following equation:
                              ω          e                =                                            θ                              1                ⁢                                                                  ⁢                iTsh                                      -                          θ                              2                ⁢                                                                  ⁢                iTsh                                                                        T              sh                        -                          τ                              1                -                2                                                                        (        5        )                            where θ1iTsh and θ2iTsh represent phase angles of a first short-circuit current vector and a second short-circuit current vector in the α-β static coordinate system respectively, and τ1-2 is a time interval between the first short circuit and the second short circuit.        
An angle θIdq between iTsh and the d-axis in the d-q coordinate system is calculated according to a following equation:
                                          θ            Idq                    ≅                      arctan            ⁡                          (                                                i                  q                                                  i                  d                                            )                                      =                              arctan            ⁡                          (                                                                    -                                                                  φ                        f                                                                    L                        d                                                                              ⁢                                      sin                    ⁡                                          (                                                                        ω                          e                                                ⁢                                                  T                          sh                                                                    )                                                                                                            -                                                                  φ                        f                                                                    L                        d                                                                              ⁢                                      (                                          1                      -                                              cos                        ⁡                                                  (                                                                                    ω                              e                                                        ⁢                                                          T                              sh                                                                                )                                                                                      )                                                              )                                =                      arctan            ⁡                          (                                                                    L                    d                                    ⁢                                                                          ⁢                                      sin                    ⁡                                          (                                                                        ω                          e                                                ⁢                                                  T                          sh                                                                    )                                                                                                            L                    q                                    ⁡                                      (                                          1                      -                                              cos                        ⁡                                                  (                                                                                    ω                              e                                                        ⁢                                                          T                              sh                                                                                )                                                                                      )                                                              )                                                          (        6        )            
The information on the rotor position is:θe=θIdq−θfαβ  (7)
In the above method, the information on the rotor position is calculated by using a two short-circuiting method. However, the method relies on the inductance parameter and the rotor flux linkage parameter of the permanent magnet synchronous motor, and thus an error of the inductance parameter and the rotor flux linkage parameter will have a serious impact on the accuracy of the information on the rotor position.